Method of making high-grade borehole casings



Patented 0.1.24, 1933 man STATES PATENT? OFFICE METHOD OF MAKING HIGH-GRADE BORE- HOLE CASINGS No Drawing. Application October 14,

iegal No. 568,865, and in Germany October This invention relates to a method of making high-grade borehole casing.

The borehole casings hitherto employed in the oil industry have been manufactured from an-= healed carbon steel with a yield point of about lag. per sq. mm., a tensile strength of kg. per sq. and an elongation of about 18%. With the continued increase in the depth of boreholes, these tubes are no longer able to meet the considerably increased stresses, being liable to kink, becoming crushed by the rock pressure or fracturing at the jointing sockets. The most obvious course of employing harder carbon steels is out of the question, since, although the increase in the carbon content raises the yield point w and resistance to vibration, it considerably impairs the elongation and tenacity. In view of the limitation of weight, increasing the thickness of the tube walls must also be discarded.

On this account, it has been proposed to emm ploy alloyed steels, especially nickel steels, for such high-grade borehole casings. However, apart from their cost, it is necessary, for the attainment of their optimum mechanical properties, that these steels should be normalized, that is to say, quenched in water or oil and then tempered. Apart from its high cost, quenching is always attended with more or less distortion .of the tubes, especially in the case of long lengths, and they must therefore be trued up, after norm malizing, thereby entailing a certain unfavourable effect on their mechanical properties and additional expense.

Normalizing improves the yield point and ten-- sile strength without appreciably lowering the tenacity. The resistance to vibration also increases in accordance with the tensile strength, but normalized steels are known to be more sensitive to notching than annealed steels. This is manifested, for example, in that the resistance to oscillation of normalized steels is diminished to a far greater extent by the occurrence of slight superficial injuries to the test bar than is the case with annealed steels. Consequently, the difflcultles in manufacturing such high-grade borehole casings are very considerable.

resistance to vibration and good tenacity can be produced by employing a steel of the following composition:-0.l to 0.4% of carbon, 0.? to 1.5% of silicon, 0.9 to 2.0% of manganese, 0.; to 2.0% of copper, 0.2 to 1.0% of chromium and 0.1 to 0.5% of aluminium.

The tubes made from this steel are annealed or preferably rolled in the heat and then tempered at low temperatures up to about 500 C.. for a short time, in order to allow freedom of expansion and to impart optimum mechanical properties.

Borehole casings made from steel with 0.18% C, 0.8% Si, 1.0%Mn, 0.9% Cu, 0.4% Cr, and 0.2% A1, exhibited the following mechanical properties:--

Yield point kg. per sq. mm., tensile strength 69 kg. per sq. mm., elongation 22%. reduction of area 58%, notched-bar tenacity 9 mkg. per sq. cm., resistance to oscillation 40 kg. per sq. mm.

Consequently, the borehole casings produced in accordance with the invention possess such a heightened yield point and tensile strength that the permissible stresses can be increased by more than 60%. Moreover, the tubes are not more sensitive to notching than annealed carbon steels, and are far superior, in this respect, to all normalized tubes.

I claim High-grade borehole casings consisting of a steel containing 0.1 to 0.4% of carbon, 0.! to 1.0% 01' silicon, 0.9 to 2% of manganese, 0.2 to 0.9% of chromium, 0.7 to 1.5% 01 copper, and 0.1 to 0.2% or aluminium, in a tempered condition, said steel possessing approximately the following properties: yield point 55 kg. per sq. mm., tensile strength 69 kg. per sq. mm., elongation 22%, reduction of area 58%, notched-bar tenacity 9 mks. per sq. cm., resistance to oscillation 40 kg. per sq. mm.

HERBERT BUCHHOLTZ. 

